The present invention relates to a process for producing a mold intended for the molding of at least a portion of a tread of a tire. More particularly, it concerns a process for molding the mold of a tire on a countermold, said countermold being provided with separately made elements in relief which protrude from the molding surface of said countermold. The portion which protrudes is intended to be inserted and anchored in the mold during the pouring of said mold.
When it is desired to produce a tread pattern of a tire comprising one or more fine slits, that is to say relatively deep slits of a thickness less than or equal to 2.5 mm, elements in the form of metal blades are produced separately before incorporating them in the mold for said tread; the blades are in general of steel so as to have sufficient rigidity to permit a large number of moldings of tires without change in the geometry of said blades.
French patent 2 430 838 describes, for the production of a mold which has blades, a process which consists in machining a countermold on a lathe and then producing fine slits in said countermold by means of a spark erosion process, each slit having a geometry which corresponds precisely to the geometry of the blade which is intended to be inserted therein before proceeding with the pouring of aluminum for forming the mold.
Such a process permits the putting in place only of blades of removable shape, since once the casting has solidified, the blades are partially inserted in the mold and their extraction from the countermold can be effected only by relative sliding in their respective housings in the countermold in a predetermined direction of removal.
The improvement in the performance required from tires has led to the conceiving of tire tread patterns containing fine slits which have large variations in geometry in the thickness of the tread of said tire. One example of this type of slit is described, for instance, in French patent application FR 2 641 501. In such cases, the blades have non-removable shapes which makes the process described in French patent 2 430 838 entirely inoperative. It should be noted that, in connection with the molding of a tire tread, the removal of the blades from the mold is possible due to the deformability and elasticity of the materials constituting said tread.
French patent FR 1 203 290 describes another process for manufacturing a tire mold by molding on a plaster countermold in which metal blades are partially embedded at the time of the production of the countermold. After solidification of the pouring on said plaster countermold, the blades which are firmly anchored in the mold are freed from the plaster countermold by breaking the latter, which of course makes it impossible to use the countermold for another molding. While this process makes it possible readily to produce a mold having non-removable blades, it is unfortunately a lengthy one to carry out and is particularly expensive since it requires the production of a new plaster countermold upon each molding of a tire mold.